This invention relates to heat exchangers, and in particular, to air cooled exchangers for cooling viscous fluids such as automotive engine oils, transmission fluid and power steering fluid.
In the past, heat exchangers employed for liquid-to-air heat exchange of high viscosity/low thermal conductivity fluids such as engine oil, transmission fluid, transaxle fluids or hydraulic fluids have been commonly produced in three main designs. The first design is an extruded tube and fin design wherein one or more tubular channels is extruded with integral internal fins. A difficulty with this design is that the heat transfer per volume of fluid flowing through the exchanger is usually relatively low, although the flow resistance or pressure drop through the exchanger also tends to be relatively low. There is also a practical limitation as to the depth of the integral internal fins in the tubes that can be extruded and the weight of this type of exchanger is relatively high.
The second common design consists of a bank of extruded or weld-seam tubes with expanded metal turbulizers located inside each tube and exterior cooling fins located between and in contact with the exterior of the tubes. This type of heat exchanger generally exhibits higher heat transfer due to the greater liquid flow turbulization by the turbulizer inside the tubes, however, the flow resistance or pressure drop in the liquid flow through the tubes is undesirably high, and the use of a turbulizer naturally increases the manufacturing costs of the heat exchanger.
The third common design for these liquid-to-air heat exchangers is a plate and fin design in which an expanded metal turbulizer is installed between a pair of mating elongate plates. Again, this type of heat exchanger produces undesirably high liquid flow resistance and the manufacturing cost is high because of the extra steps involved in inserting the turbulizer and the necessity of ensuring that a good bond is achieved between the turbulizer and the plate.
Plate and fin type heat exchangers without turbulizers have been used in other applications, such as automotive air conditioning evaporators. An example of such a device is shown in U.S. Pat. No. 4,470,455 issued Sept. 11, 1984 to DEMETRIO B. SACCA. This patent shows a heat exchanger formed of a plurality of stacked pairs of plates, the plates having rows of overlapping ribs angled obliquely to the flow path. This provides a circuitous or tortuous flow path through the plate pair. While this may be good for the evaporation of refrigerant, it would not be acceptable for high viscosity/low thermal conductivity fluids such as engine oils or hydraulic fluids, because the pressure drop through this type of exchanger would be unacceptably high.
Another example of an automotive air conditioning evaporator using stacked plate pairs without a turbulizer is disclosed in U.S. Pat. No. 4,600,053 issued July 15, 1986 to R. L. PATEL et al. This patent shows a plurality of rows of overlapping dissimilar mating beads said to increase the heat transfer co-efficient of the heat exchanger. Again, however, since this is an air conditioning evaporator for vaporizing refrigerant, flow resistance and pressure drop is not a major concern. This type of heat exchanger could not be used for high viscosity/low thermal conductivity fluids such as engine oils or hydraulic fluids, again because the pressure drop through the exchanger would be unacceptably high, or in other words, the heat transfer efficiency of the exchanger would be unacceptably low. Also, the dissimilar mating beads would not produce sufficient vorticity or turbulence for engine oils and hydraulic fluids.